There has been conventionally known a wavelength conversion laser for outputting light while converting the wavelength of fundamental wave laser light into that of a converted wave such as a second harmonic (second harmonic generation: SHG), a sum frequency wave or a differential frequency wave by using nonlinear optical phenomenon of wavelength conversion element.
A wavelength conversion laser is, for example, provided with a fundamental wave laser light source 101, a lens 102 for condensing fundamental wave laser light emitted from the laser light source 101, a wavelength conversion element 103 for converting the condensed fundamental wave laser light into a second harmonic and a dichroic mirror 104 for separation into fundamental wave laser light and harmonic laser light as shown in FIG. 11.
The wavelength conversion element 103 is made of a nonlinear optical crystal and wavelength-converts the fundamental wave. Specifically, the wavelength conversion element 103 has a crystal orientation, a polarization-reversed structure and the like appropriately adjusted such that the phases of the fundamental wave and the converted wave match. Particularly, a wavelength conversion element having a polarization-reversed structure is capable of highly efficient wavelength conversion even with low power because of quasi-phase matching and can perform various wavelength conversions depending on its design. The polarization-reversed structure is a structure formed with regions where the spontaneous polarization of the wavelength conversion element 103 is periodically reversed.
A conversion efficiency η used for conversion from a fundamental wave into a second harmonic can be expressed as follows if it is assumed that L denotes an interaction length of the wavelength conversion element, P the power of the fundamental wave, A a beam cross-sectional area in the wavelength conversion element and Δk a deviation from a phase matching condition.η∝L2P/A×sin c2(ΔkL/2)
In the above equation, upon a deviation from the phase matching condition, the conversion efficiency decreases to reduce the generation of the second harmonic (converted wave). Thus, a control is executed to set the temperature of the nonlinear optical crystal to a specified temperature in a permissible range so that no deviation from the phase matching condition occurs.
For example, it has been proposed to control the driving of a temperature regulator so that the light intensity of a converted wave converges to a target value using a detector for detecting the light intensity of the converted wave and the temperature regulator for a nonlinear crystal as disclosed in patent literature 1.
According to a construction of patent literature 1, it is possible to obtain a high conversion efficiency and control the output of a wavelength conversion laser.
However, it is not proposed in patent literature 1 to control an intensity distribution of the converted wave emitted from the wavelength conversion laser by controlling the wavelength conversion efficiency.
[Patent Literature 1]
Japanese Unexamined Patent Publication No. H04-318528